Computer systems and other electrical systems typically use one or more buses to interconnect integrated circuits and other system components. Data, typically in digital form, is communicated between such circuits and components via a bus.
Recent trends in bus development have dramatically reduced the voltage swings associated with different data states on the bus. Early bus systems saw rail-to-rail voltage swings from 3.5 or 5.0 volts down to zero volts. More contemporary bus systems provide voltage swings of less than 1 volt. Limited voltage swings have resulted in reduced power dissipation and lower levels of induced noise on the bus. These reductions are particularly important in the context of bus systems running at ever increasing clock rates.
However, reduced voltage swings and increasing data rates pose considerable problems to the system designer. Reduced voltage swings necessarily provide reduced voltage margins. That is, the ability of system components to distinguish one data state from another on the bus is reduced as the upper and lower swing thresholds move closer together. Similarly, increasing operating frequencies require system components to detect data on the bus during shorter and shorter time intervals. Accordingly, voltage and timing margins for bus signals are often limiting factors in determining overall system performance.
FIG. 1 is a simple block diagram of a bus system comprising a master 11 and slaves 12a-12n connected via a data bus 30 and a folded clock signal (Clock-To-Master “CTM” and Clock-From-Master “CFM”). As shown in the related signal timing diagram of FIG. 2, valid data is apparent on the data bus during a period T. Ideally, the data signal would be clearly defined at VHI for a first data state and at VLO for a second data state. VHI and VLO would be equally spaced above and below a reference voltage, Vref. Such ideal relationships would provide maximum voltage margin between high and low data states, and correspondingly optimal signal detection capabilities for system components.
Additionally, data would be detected at time t1 during the “data eye,” i.e., the period (“tbit”) during which valid data is on the bus between data transition periods. Time t1 corresponds to center of the data eye and provides maximum timing margin (½ tbit) for data detection between data transition periods.
Unfortunately, the ideal voltage and timing margins illustrated in FIG. 2 do not exist in actual bus systems. FIGS. 3A and 3B illustrate typical timing skews between the ideal occurrence of a clock signal within the data eye and actual occurrences. Such clock timing skews arise from many possible sources, such as differences in the manufacture of signal line traces on printed circuit boards, or differences in signal flight time due to the buildup of standing waves induced by signal and clock reflections and noise on signal lines.
In FIG. 3A, a CTM signal transition occurs early in the data eye by a period δ before the ideal placement of the CTM transition. In other words, the master reads data from a slave too early relative to the clock by a time δ. In FIG. 3B, the CFM signal occurs late in the data eye by the same time. Thus, the master writes data into the slave too late in the data eye by time δ.
Slight differences in the actual timing of the clock signal and/or the data signal will result in a shift of their ideal timing relationship. In addition, the bidirectional nature of some signal lines in the bus system will result in timing shifts of different polarities depending on the direction of data flow.
Each one of the multiple slaves connected to the bus might have a different and unpredictable timing error in relation to the ideal clock placement. Such errors reduce the overall timing margin in the system. Further, as actual transition times wander, the hazard arises that a device will attempt to read data during a data transition period, i.e., during a period where the data is not valid on the bus. This hazard increases with system operating frequency.
A comparison between FIGS. 4A and 4B illustrates the problem of voltage errors in the bus system of FIG. 1. In FIG. 4A, ideal relationships between VHI, VLO, and Vref are shown, where the voltage swing from VHI to Vref is the same as the voltage swing from Vref to VLO. In one embodiment, VHI is 1.8 volts, Vref is 1.4 volts, and VLO is 1.0 volts. In contrast, the voltage swing from VHI to Vref in FIG. 4B is much less than the voltage swing from Vref to VLO. Such an unequal relationship dramatically reduces the voltage margin for accurately detecting a data value on the bus associated with VHI.
Where the bus system of FIG. 1 uses single-ended data and a single reference voltage, as explained below in greater detail, the presence of voltage errors is particularly harmful. For such systems, a reference voltage (Vref) centered between VHI and VLO, such as shown in FIG. 4A, would provide maximum voltage margin.
Unfortunately, a number of system phenomena prevent the stable, centered positioning of VHI and VLO about Vref. For example, channel-DC resistance induces voltage errors in current mode signaling systems. With channel-DC resistance, a write data eye can shift in voltage as it goes down the signaling channel. That is, slaves further away from the master are likely to experience smaller voltage swings than the swings of slaves closer to the master, simply due to increasing channel-DC resistance which forms a voltage divider with the termination resistance. In addition, setting output voltage levels to be symmetric about Vref in manufactured systems which experience real process, voltage, and temperature variations is very difficult.
At some point, like the timing errors illustrated above, voltage errors will result in a data error. At a minimum, the presence of voltage errors will reduce the voltage margins allowed within a bus system.
Recognizing the inevitable degradation of the timing and voltage characteristics of bus system signals and the problems associated with same, conventional bus systems sought to compensate for the timing and voltage errors by gross adjustments of the data and/or clock signals in the master. This approach improved signaling margins where degradations were predictable, or where a very limited number of components were connected to a simple bus. However, as bus systems have increased in complexity and size, it has become clear that many factors adversely impacting timing and voltage margins are unique to individual slave devices, or to the relative position of the slave to the master within the overall system.
Thus, the conventional use of timing and voltage offsets in the master has proven ineffective in contemporary bus systems. Similarly, the use of vernier re-calibration techniques has resulted in inconsistent system performance and unacceptable bandwidth degradation in high frequency systems. Accordingly, a need remains for an approach to timing and voltage error compensation which is reliable and well adapted to complex, high frequency bus systems.